Protein isolates (PI) were obtained from pine kernel meal (PKM) and dephenolized pine kernel meal (DPKM) by alkali dissolution followed by acid precipitation, and globulin (Glo), albumin (Alb) and gluten (Glu) were prepared from PKM and DPKM by Osborne fractionation method. The effects of endogenous polyphenols on the physicochemical and functional properties and structure of these proteins were explored. It was shown that after the removal of phenolics, the contents of phenolic compounds, ash and carbohydrates decreased, while the protein content significantly increased (P < 0.05). The contents of total and free sulfhydryl groups in the proteins from DPKM were significantly higher than those in the proteins from PKM (P < 0.05), whilst the opposite was observed for the content of disulfide bonds and the surface hydrophobicity. Higher solubility, emulsion stability, foaming capacity and foam stability were observed in the proteins from DPKM, while the emulsifying capacity decreased after the removal of phenolic compounds (P < 0.05). The secondary structure of pine kernel proteins was changed by the removal of polyphenols. Dephenolization could alter the secondary structure of pine kernel proteins. The proteins from both PKM and DPKM had typical absorption peaks, and the positions of the amide Ⅰ and amide Ⅱ bands were shifted. In addition, the proportion of β-sheet increased, while the proportions of α-helix, β-turn and random coil decreased in DPKM proteins compared with PKM proteins. Fluorescence spectra showed that polyphenols had a fluorescence quenching effect on PI, Alb and Glu, but not on Glo. The structure and properties of pine kernel proteins can be significantly affected by dephenolization, and this treatment method can be selected according to the application purpose.

Phenolics were extracted from the by-products (leaves, green leafy covers, shells, branches and seed coats) of the hybrid hazelnut Corylus heterophylla Fisch. × Corylus avellana L. using four different polar solvents (water, methanol, 80% ethanol and ethyl acetate) and their antioxidant, hypoglycemic and anti-inflammatory activities were evaluated in vitro. The phenolics were identified by ultra-high performance liquid chromatography-mass spectrometry (UPLC-MS). The leaf and branch extracts showed stronger antioxidant, hypoglycemic and anti-inflammatory activities in vitro than the extracts from green leafy covers, shells and seed coats. The biological activities were largely correlated with total phenols and proanthocyanidins in the by-products of hazelnut. The methanol extract of hazelnut branches had the highest content of total phenols (calculated as gallic acid equivalent per dry mass) and procyanidins (calculated as proanthocyanidin equivalent per dry mass), which were (37.13 ± 0.22) and (2.27 ± 0.04) mg/g, respectively. The content of total flavonoids (calculated as rutin equivalent per dry mass) in the 80% ethanol extract of hazelnut leaves was the highest, which was (18.21 ± 1.37) mg/g. For each plant part, the contents of total phenols, total flavonoids and proanthocyanidins in the ethyl acetate extract were lower than those in the other solvent extract, but the ethyl acetate extract inhibited the activities of α-amylase and lipoxygenase significantly (P < 0.05). The ethyl acetate extract of green leafy covers had the best inhibitory effect on α-amylase, while the ethyl acetate extract of leaves showed the strongest inhibitory activity on lipoxygenase. According to UPLC-MS analysis, the majority of phenolic compounds in hybrid hazelnut by-products were flavonoids along with many bioactive compounds such as terpenes, alkaloids, lignans and organic acids. Bioactive ingredients in hybrid hazelnut by-products have the potential to be used in functional foods or pharmaceuticals.